\documentclass[a4]{parsibook}

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\begin{document}

\begin{equation*}
T(n)= \left\lbrace
\begin{array}{ll}
T(\frac{n}{2})+1 & $  اگر $n$ زوج است $\\
2T(\frac{n-1}{2}) & $ اگر $n$ فرد است$
\end{array}\right.
\end{equation*}

\end{document}